This invention relates generally to the field of heating, ventilating, and air conditioning (HVAC) systems and, particularly, to air handling products with an airside economizer, a return fan and a building pressure control requirement. Embodiments of the present invention apply to controls for air handling units with a constant volume or variable volume supply fan, a variable volume return fan, and a central exhaust that are used in commercial rooftop products or other HVAC products with similar design characteristics.
Generally, HVAC systems are used to heat and/or cool building spaces. Such building spaces include single story and multi-story schools, office buildings, and manufacturing facilities, for example. These systems may be unitary or built-up systems, and may be used to condition the air in multiple zones or building spaces.
The HVAC system is a heating/cooling circuit. The HVAC system can comprise: heating or cooling coils for conditioning air, a supply fan to supply air through a supply duct to a building space to be conditioned, a return fan that draws return air through a return duct from a conditioned building space and blows the return air to an exhaust path and/or to a downstream return duct that supplies the return air back to a filter upstream of heating or cooling coils and the supply fan. The return air may also be mixed with outside air used for economizer (“free”) cooling that is drawn into an outside air duct connected to the outside and return air mixing box. Other configurations are known. For example, instead of employing a return fan, a relief fan or gravity relief may be employed. In addition, various dampers, pressure gauges, temperature gauges, and motor drives have been employed.
The HVAC system has the complex task of controlling the flow of air fluid through each fluid flow path, e.g., the supply flow path, the outside flow path, the return flow path, and the exhaust or relief flow path. One HVAC system that has been employed is a constant air volume system. A constant air volume system delivers supply air at a constant rate to the space to be conditioned.
Another system that has been employed is the variable air volume system. This system delivers supply air at a variable rate to the space to be conditioned, and has been typically employed due to its capability of improving energy efficiency. When such constant and variable volume systems utilize a return fan as well as a supply fan, for example, such systems provide reasonably good control of the ventilation requirements, pressurization of the building, and relative control of the minimum outside air supplied to the building.
Utilizing a return fan in HVAC systems can help in building pressure control. An HVAC system utilizing a return fan can ensure sufficient return plenum pressure by overcoming the wide-open exhaust damper and exhaust/relief path losses. In addition, a return fan can ensure adequate fan capacity to overcome the return path losses.
For example, supply fans that have been used in conjunction with return fans have been controlled as a function of the HVAC system static pressure. The supply and return air flows are typically volumetrically synchronized to control the building pressure and to control the minimum outside air provided to the building to meet ventilating code requirements. In such systems, the supply and return fans are controlled so that a pre-selected flow rate differential between the return fan and supply fan is maintained, with the return fan having a lower flow rate than the supply fan. The pre-selected flow rate differential between the supply fan and return fan corresponds to the flow of air exhausted from the building to satisfy code requirements. The return fan output is directed back to the input of the supply fans and the difference in flow rate between the supply fan and return fan is made up by connecting an outside air duct to the input of the supply fan, thereby, allowing outside air to flow into the supply fan at a rate approximately equal to the differential between the flows through the supply fan and return fan. The mixture of recirculated and outside air provided to the supply fan may be tempered—heated or cooled—at the input or the output of the supply fan. This arrangement, however, does not provide direct control of building pressure.
Alternatively, a basic method for controlling a return fan in such systems is described in relation to Arrangement 1 in American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Guideline 16-2003 (“Guideline 16-2003”). Supply fan capacity control provides supply duct pressure control, return fan capacity control provides return plenum pressure control, and exhaust damper position control provides building pressure control. This arrangement, therefore, intends to provide direct control of building pressure. Guideline 16-2003 prescribes a strategy to maintain a constant return plenum pressure at all operating conditions. This strategy is described to prevent excessive control loop interaction. But, to maintain decoupling and control stability, the Guideline recommends a difference between the closed-loop response time of the building pressure and return fan functions of at least 5:1.
The current control methods require the return fan to meet a maximum plenum pressure requirement when at the design supply air flow and exhaust air flow rates with a one hundred percent (100%) outside air flow and a zero percent (0%) return air flow. At all other exhaust part-load conditions such as with intermediate return/outside air damper positions, throttled supply air flow and/or for the minimum plenum pressure modes with a one hundred percent (100%) return air flow and a zero percent (0%) outside air flow, the return plenum pressure is controlled to the same maximum pressure setpoint and is not controlled to a lower pressure.
Significant non-linear effects in the system can exist across the range of operating conditions, due to the damper control components and the inherent pressure versus flow relationship in fans, coils, and duct system components. These non-linearities, combined with variable air flow rates, cause significant process gain variation for each controller employed in the HVAC system. That is, when using a conventional, linear PI controller with fixed gain parameters, large variation in the closed-loop response times result. This variability makes it difficult, if not impossible practically, to maintain a 5:1 response time decoupling ratio as prescribed by the Guideline 16-2003.
While in some applications the return plenum pressure can be controlled to a constant pressure (such as a few tenths to one half inches water column), frequently constant pressure should not be used when design compromises and exhaust damper size limitations result in less than ideal air flow characteristics for the exhaust dampers and relief path. Such design tradeoffs and size limitations most frequently occur with packaged HVAC systems, although similar limitations can arise with built-up systems. That is, the resulting pressure loss characteristics of the exhaust path can be much higher than the preferred drop of a few tenths inches water column. In fact, the pressure drop can be as high as or exceed 1.0 inches water column (249.089 Pa) or more. Consequently, in practice, controlling the return plenum pressure to a constant, fixed setpoint value, such as 1.0 inches water column (249.089 Pa), does not work effectively. Such a setpoint value causes balance problems between return air and mixed air pressures.
In particular, excessive return pressure from the return fan disrupts normal system operation. This is especially apparent in minimum ventilation modes with a fixed outside damper position, because the normally negative mixed air pressure will increase and cause a reduction or reversal of outside air intake.
In addition, excessive return pressure beyond the level necessary to provide the required air flow rate reduces energy efficiency. Since the supply fan and return fan are typically operated at variable speeds, with the drive frequency to these being periodically adjusted to control the volume output of each fan, the power consumed by the fans may often be in excess of that necessary to drive the volume of air that the HVAC system requires. Normal variation in thermal loads and ventilation requirements typically drive the variation in supply fan and return fan operating conditions. Other variations in the air flows through the supply fan and return fan can occur due to infiltration or exfiltration of outside air into the building through vents, cracks, etc. Since the outside air infiltrating the building is not conditioned to a set temperature, and is generally either too warm or too cool, the introduction of excess outside air can result in further energy inefficiencies. The introduction of too little outside air can also result in indoor air quality problems.